The present invention relates to a ceramic heating element and, more particularly, to a zirconia fiber-based or zirconia fiber-reinforced zirconia heating element which generates heat by passing a current or by induction heating, and a process therefor. The present invention also relates to a heating structure composed of such a zirconia heating element and a lead member for passing a current and a process therefor.
Heretofore, the heating elements used in oxidizing atmospheres have varied depending upon the temperatures used. In general, SiC heating elements have been used at temperatures up to about 1,400.degree. C., and MoSi.sub.2 heating elements have been used at temperatures up to 1,700.degree. C. Heating elements capable of being used at higher temperatures have not been widely utilized. While heating elements of La.sub.2 O3-Cr.sub.2 O.sub.3 system have been partially used, it is not widespread because La.sub.2 O.sub.3 is expensive, is difficult to prepare a large sintered body and because Cr.sub.2 O.sub.3 evaporates with use of the heating elements, to contaminate the inner portion cf a furnace.
On the other hand, there have been developed resistance heating units composed of negative characteristic elements exhibiting electric conductivity in the vicinity of 1,000.degree. C. by heating ceramic materials containing zirconia (ZrO.sub.2) or thoria (ThO.sub.2) and other additives such as calcia (CaO) or yttria (Y.sub.2 O.sub.3). (Japanese Patent Publication No. 12330/1963 or the like). Ultra-high temperature furnaces composed of resistance heating units have been already partially practically utilized It is expected that resistance heating units will become widely utilized to produce high melting point single crystals, to produce special high temperature materials, or to study physical properties at high temperatures.
Of these, zirconia heating elements are heating materials obtained by adding a small amount of specific oxides to zirconium oxide (ZrO.sub.2) and sintering the mixture at a high temperature Because the melting point of zirconia (ZrO.sub.2) is 2,690.degree. C., zirconia heating elements can afford high temperature of up to 2,400.degree. C.
However, the prior zirconia heating elements often fail due to the fact that current is locally passed. Their heat conductivity is low and their coefficient of expansion is large. Accordingly, the prior zirconia heating elements exhibit low thermal shock resistance and tend to be damaged by shock. Accordingly, zirconia heating elements have not been substantially produced.
Further, heretofore, in order to form zirconia heating structures from these zirconia heating elements, the following methods have been utilized: a method wherein pores are formed in said heating element body, platinum wires passed through the pores as lead members for passing a current, and entwined to fix them; another method wherein a platinum paste is applied to the heating element body and this heating element body is joined to lead members for passing a current at diffusion areas as wide as possible.
However, the former method poses a relaxation problem in use, whereas the latter method puts large restrictions on the form of the furnaces. Further, these methods can fail due to the fact that a current is locally passed through the contact portion between the lead member for passing a current, and the heating element body.
There has also been proposed a method of securing a heating element body and a lead member for passing a current by means of a castable refractory. However, when both members are secured by means of the prior castable refractory, they cannot follow in thermal expansion or shrinkage generated by passing a current, and the contact resistance associated with the generation of cracks is increased. Thus, the lead member for passing the current is overheated and damaged, and therefore the prior product cannot be used as a heating element in many cases.
An object of the present invention is to provide a zirconia heating element which generates heat by passing a current or by induction heating and a process therefor, wherein the zirconia heating element has excellent exothermic characteristics when a current is passed and wherein it has resistance to thermal shock.
Another object of the present invention is to provide a process for producing a zirconia heating element which has excellent current-passing exothermic characteristics or induction heating characteristics and resistance to thermal shock wherein the variation of these characteristics is little if damage should occur, and wherein the zirconia heating element can be readily produced.
A further object of the present invention is to provide a practical zirconia heating structure obtained by joining a heating element body and a lead member for passing a current using a composition exhibiting conductivity at a high temperature which can strongly join the zirconia heating element electrically and mechanically and the lead member for passing a current through the zirconia heating element wherein the lead member has sufficient conductivity even at room temperature; and a process therefor.